﻿2 
  KEPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SUEVEY. 
  

  

  tions 
  have 
  been 
  carefully 
  connected 
  in 
  height; 
  by 
  fore 
  and 
  back 
  angles 
  

   of 
  elevation 
  and 
  depression, 
  to 
  be 
  used 
  as 
  a 
  check 
  on 
  the 
  barometric 
  

   heights, 
  while 
  the 
  heights 
  of 
  ail 
  located 
  points 
  have 
  been 
  determined 
  by 
  

   a 
  system 
  of 
  angles 
  of 
  depression 
  and 
  elevation. 
  

  

  The 
  district 
  assigned 
  to 
  this 
  division 
  for 
  the 
  summer 
  of 
  1875 
  joined 
  on 
  

   to 
  the 
  south 
  borders 
  of 
  that 
  surveyed 
  in 
  1873 
  and 
  1874. 
  Longitude 
  

   104° 
  30' 
  formed 
  the 
  eastern, 
  longitude 
  108° 
  the 
  western, 
  and 
  36° 
  45' 
  

   north 
  latitude 
  the 
  southern 
  boundaries. 
  

  

  A 
  plan 
  for 
  the 
  most 
  rapid 
  and 
  successful 
  completion 
  of 
  the 
  work 
  un- 
  

   dertaken 
  was 
  prepared 
  by 
  Mr. 
  Wilson, 
  and 
  subsequently 
  carried 
  out 
  as 
  

   proposed. 
  This 
  district 
  contained 
  the 
  foot-hills 
  sipping 
  eastward 
  from 
  

   the 
  Front 
  Range, 
  the 
  southern 
  continuation 
  of 
  the 
  Saiigre 
  de 
  Cristo 
  

   Kange, 
  the 
  southern 
  end 
  of 
  San 
  Luis 
  Valley, 
  the 
  extension 
  of 
  the 
  La 
  

   Plata 
  Mountains, 
  and 
  the 
  lower 
  country 
  of 
  the 
  Eio 
  San 
  Juan 
  and 
  its 
  

   tributaries. 
  A 
  small 
  portion 
  of 
  the 
  sedimentary 
  eastern 
  foo:-hills 
  was 
  

   first 
  surveyed, 
  and 
  the 
  work 
  then 
  carried 
  westward 
  to 
  the 
  mountainous 
  

   vicinity 
  of 
  the 
  Upper 
  Rio 
  Grande. 
  Instead 
  of 
  forming 
  a 
  well-defined, 
  

   sharply-limited 
  range, 
  the 
  mountains 
  south 
  of 
  the 
  Rio 
  Grande 
  have 
  the 
  

   form 
  of 
  a 
  high 
  plateau, 
  with 
  numerous 
  isolated 
  peaks. 
  Both 
  the 
  plateau 
  

   and 
  the 
  peaks 
  mentioned 
  are 
  volcanic. 
  From 
  the 
  position 
  of 
  volcanic 
  

   beds 
  composing 
  the 
  higher 
  peaks, 
  it 
  may 
  be 
  inferred 
  that 
  at 
  one 
  time 
  the 
  

   summit 
  of 
  the 
  plateau 
  extended 
  to 
  a 
  considerably 
  higher 
  altitude 
  than 
  

   at 
  present. 
  Toward 
  the 
  southwest, 
  it 
  drops 
  off 
  suddenly 
  into 
  the 
  lower 
  

   country, 
  containing 
  the 
  Rios 
  Piedra 
  and 
  Pinos, 
  presenting 
  aline 
  of 
  steep, 
  

   rough 
  mountains, 
  formed 
  in 
  part 
  by 
  the 
  abrupt 
  termination 
  of 
  the 
  plateau, 
  

   in 
  part 
  by 
  the 
  peaks 
  above 
  mentioned, 
  the 
  former 
  contrasts 
  strongly 
  with 
  

   the 
  rich 
  land 
  in 
  the 
  valleys 
  of 
  the 
  two 
  rivers. 
  Here, 
  as 
  at 
  so 
  many 
  points 
  

   in 
  the 
  districts 
  surveyed 
  by 
  the 
  southern 
  division, 
  the 
  geological 
  features 
  

   determine 
  the 
  orographic 
  character. 
  With 
  the 
  plateau 
  end 
  the 
  volcanic 
  

   beds 
  and 
  the 
  sedimentaries 
  of 
  Cretaceous 
  age 
  set 
  in. 
  But 
  few 
  stratigraph- 
  

   ical 
  disturbances 
  have 
  changed 
  the 
  relative 
  position 
  of 
  the 
  beds, 
  and 
  

   the 
  country 
  therefore 
  shows 
  regular 
  features. 
  Long 
  lines 
  of 
  high 
  

   ridges, 
  abrupt 
  on 
  the 
  north 
  side, 
  sloping 
  more 
  gently 
  toward 
  the 
  south, 
  

   extend 
  from 
  east 
  to 
  west, 
  and 
  are 
  cut 
  by 
  the 
  drainage 
  of 
  the 
  San 
  Juan. 
  

   Eastward, 
  the 
  edge 
  of 
  the 
  plateau 
  recedes, 
  losing 
  at 
  the 
  same 
  time 
  some 
  

   of 
  its 
  roughness, 
  and 
  a 
  broad 
  expanse 
  of 
  comparatively 
  low 
  bluff 
  conn- 
  

   try 
  appears. 
  Rich 
  valleys, 
  partly 
  timbered 
  or 
  covered 
  with 
  grnss, 
  fol- 
  

   low 
  the 
  course 
  of 
  the 
  larger 
  streams, 
  owing 
  their 
  formations 
  to 
  the 
  rapid 
  

   erosions 
  and 
  ready 
  disintegration 
  of 
  the 
  shales 
  belonging 
  to 
  Cretaceous 
  

   No. 
  2. 
  Springs 
  containing 
  an 
  unusual 
  amount 
  of 
  mineral 
  ingredients, 
  

   some 
  of 
  them 
  hot, 
  occur 
  in 
  these 
  valleys. 
  Owing 
  to 
  the 
  slight 
  south- 
  

   erly 
  dip 
  of 
  the 
  Cretaceous 
  beds, 
  this 
  formation 
  claims 
  a 
  considerable 
  area 
  

   of 
  the 
  region 
  extending 
  from 
  the 
  Rio 
  Animas 
  eastward 
  to 
  the 
  border 
  of 
  

   the 
  district. 
  Above 
  the 
  well-determined 
  strata 
  of 
  Nos. 
  2 
  and 
  3, 
  a 
  series 
  

   of 
  shales 
  and 
  sandstones 
  set 
  in, 
  in 
  which 
  no 
  characteristic 
  fossils 
  what- 
  

   ever 
  w6re 
  found. 
  They 
  reach 
  a 
  thickness 
  of 
  about 
  3,000 
  feet, 
  and 
  con- 
  

  

  